1. Field of the Invention
The present invention relates to an automotive traction control system which is applied for suppressing excessive driving force exerted on at least one of driven wheels during quick depressing operation of the accelerator pedal, such as quick starting, quick acceleration, or the like, and specifically to an automotive traction control system employing a brake fluid pressure actuator applied commonly to a traction control executed during acceleration-slip and an anti-skid brake control executed during deceleration-slip.
2. Description of the Prior Disclosure
Recently, there have been proposed and developed various automotive traction control systems, namely a traction control system generally abbreviated as a "TCS" which is provided for suppressing excessive driving force exerted on driven wheels during acceleration-slip, an anti-skid brake control system generally abbreviated as an "ABS" which is provided for preventing brakes from locking road wheels during deceleration-slip, such as quick braking or braking on a low frictional road surface so as to provide maximum effective braking, and a servo-assisted brake control device which is provided for generating a wheel-cylinder pressure higher than a master-cylinder pressure at a relatively light brake pedal pressure. In such a conventional brake control system employing both the TCS and the ABS, a brake fluid pressure control actuator is usually arranged upstream of the wheel-cylinders to be applied commonly to the acceleration-slip control and the deceleration-slip control. One such traction control system employing the brake fluid pressure actuator applied commonly to the acceleration-slip control and the deceleration-slip control has been disclosed in Japanese First Publication Tokkai (Showa) 58-122246.
The above-mentioned conventional traction control system has a pair of switching valves, namely a first switching valve being arranged between a master cylinder and a brake fluid pressure actuator for establishing and blocking a fluid communication between the master cylinder and the brake fluid pressure actuator, and a second switching valve being arranged between the brake fluid pressure actuator and an external brake fluid pressure source provided for generating a brake fluid pressure irrespective of depression of the brake pedal for establishing and blocking a fluid communication between the external brake fluid pressure source and the brake fluid pressure actuator. Furthermore, the brake fluid pressure actuator is traditionally comprised of a plurality of pairs of fluid pressure intensifying valve and fluid pressure reducing valve, each pair being associated with either one of the wheel cylinders. In case that the acceleration-slip control terminates and a normal braking operation recovers, the first switching valve is switched from the valve closed state to the valve open state so as to establish the communication between the outlet port of the master cylinder and the inlet port of the brake fluid pressure actuator, upon termination of the acceleration-slip control, while the second switching valve is switched from the valve open state to the valve closed state so as to block the communication between the outlet port of the external brake fluid pressure source and the inlet port of the brake fluid pressure actuator. The pressure intensifying valve and the pressure reducing valve included in the brake fluid pressure actuator are both shifted from an acceleration-slip controlling state to a normal state, for permitting the master-cylinder pressure to be introduced into each wheel cylinder during the normal braking operation. Such a series of returning operations of the valves could be performed at a simultaneous timing. Therefore, the conventional traction control system has some drawbacks as hereinafter described in detail.
Assuming that switching of the second switching valve delays when compared with switching of the first switching valve, both of the switching valves are opened momentarily. In this case, there is a tendency the brake fluid pressure generated from the external brake fluid pressure source to be introduced through the second switching valve and the first switching valve to the master cylinder, in that order. There is a possibility that the master cylinder is damaged due to the pressurized brake fluid pressure created by the external brake fluid pressure source.
Assuming that the second switching valve is switched at an earlier timing than the first switching valve, both of the valves are closed momentarily. In this case, the fluid pressure stored in the brake fluid pressure actuator escapes into the wheel cylinders, thereby resulting in an increase in the wheel-cylinder pressure. As a result, there is a possibility that the driver feels uncomfortable due to undesired deceleration.
Assuming that the normal braking operation recovers just after a pressure intensifying mode of the acceleration-slip brake control according to which a high brake fluid pressure is introduced into the wheel cylinder and thereafter the control operation is shifted to the deceleration-slip control or the acceleration-slip control, there is a tendency the brake fluid to be returned from the brake fluid pressure actuator to a brake fluid reservoir to exceed a volumetric capacity of the reservoir at a pressure reducing mode of the acceleration-slip control or the deceleration-slip control. In this case, it is difficult to suitably set the pressure reducing control time. In addition, a durability of the valve may be lowered due to the remaining pressurized brake fluid having a relatively high pressure.
Assuming that the normal braking operation recovers when a predetermined period of time has elapsed after termination of the acceleration-slip brake control, a control response may be delayed by the predetermined period of time, when a stop instruction for an acceleration requirement is generated, a malfunction of the traction control system is detected, or a deceleration-slip control is required, during the acceleration-slip control. This results in an increase in braking distance during the deceleration-slip control. In other words, braking time is retarded.